Method for the hardening of wood material

Abstract: The invention relates to a method for the hardening of wood material by impregnating it with a polymerizable substance and by polymerizing the polymerizable substance by means of heat. Heat transfer and cleaning in the polymerization have been improved through the impregnated wood material being in contact with water during the polymerization. The water is preferably hot and pressurized and serves at the beginning of the polymerization as a heating medium and during the polymerization as a cooling and cleaning medium.

Description: The invention relates to a method for hardening wood material by impregnating it with a polymerizable substance and by polymerizing the polymerizable substance by means of heat. The technological use of a number of species of timber is limited by their inherent softness. The softness of wood, as well as its other mechanical strength properties, is derived from the microstructure of the wood, which is different in different species of timber. All species of timber are composed mainly of hollow cells parallel to the direction of the trunk, i.e. of wood fibers, of which both the size and the structure vary. Roughly it can be said that the larger the cell lumens in proportion to the cell size, the softer and weaker the wood. Without dealing in greater detail with the formation of the inherent properties of wood it can, however, be stated that the growth of the cell tissue is affected by, for example, the hereditary factors of the wood, the water and nutritional situation in the soil, and the climate during the growth period. A major proportion of the wood available globally consists of these so-called soft species of timber, and in most cases they have only lower-value use. This is, of course, seen in the commercial uses and prices of species of timber. Uses in which especially the hardness of wood is important include various surface materials in buildings, such as floor and table surfaces. Among the Finnish species of timber, only the birch competes in terms of its hardness on a par with foreign species, such as oak, beech, elm, etc. The hardness of wood can best be affected by filling the empty spaces in the cell tissue with a solid. The filling can be done by impregnating the wood in a closed space with a liquid which can be hardened inside the wood, for example by polymerization. One problem is how to cause the liquid to penetrate evenly into the cell tissue of the wood, since the permeability of most species of timber is poor and may vary significantly in various parts of the timber. For this reason, poorly impregnable wood is treated with an impregnation liquid in a closed space by using vacuum and/or pressure. Another problem is to cause the impregnating substances to polymerize inside the wood so that the wood is not burned or split, or does not lose properties important for its technological uses, such as its capability of being sanded, machined, glued or its surface being treated. Substances intended for the impregnation of wood are often polymerized by using heat. The reaction is exothermal and usually causes in wood materials and wood-based materials harmful splitting, changes in the dimensions, and also burning. The temperature within the wood rises to a very high level owing to the reaction. In known methods, attempts have been made to prevent this by polymerizing the pieces between hot thermal plates, as in FI Patent 44946. The thermal plates serve to initiate the reaction and also to even out the heat of reaction and cool the reaction. However, the method is space consuming and costly. Another method of controlling the polymerization reactions is to use radiation to initiate the polymerization. Pieces of wood material are in this case treated, for example, by gamma radiation. The use of the method on an industrial scale is expensive, and its implementation for sheet-like pieces of wood requires a larger apparatus configuration, thus increasing for example occupational safety risks. From FI Patent 44945 there is known a method for producing wood which contains a synthetic polymer by means of a catalyst and heat, in which method the polymerization reaction is retarded by using conjugated unsaturated fatty acids, terpenes which contain three double bonds, or derivatives of these. The use of retarders such as these has the disadvantage that they may react in different ways in different wood materials. Methods have also been used in which pieces are heated by using flowing gases and water vapor to initiate the reaction, and the same substances are used for cooling the overheating of the reaction. Cf. FI Patent 50070. However, in methods employing gas there occurs detrimental evaporation of chemicals, thus producing pieces which are hardened more unevenly. In addition, expensive gas recovery systems have to be resorted to, which increases the complexity and cost of the process. When water vapor is used, as in WO-A-83 00698, there are often produced precipitate and hardenings of polymerized plastic on the walls of the process vessels and burrs at the ends and on the sides of the pieces; with time these cause cleaning problems, operational disturbances, and problems of machining and detaching. The dimensions of the test pieces change and the incurred loss of raw material becomes relatively high. Methods have also been used in which impregnated pieces are treated with heat transfer mediums. DE Lay-Open Print 1 792 051 discloses the use of glycol as a heat transfer medium. SE Lay-Open Print 405329 discloses the use of polyethylene glycol as a heat transfer medium. In test pieces such methods result in problems associated with after-treatment, for example surface treatment. Residues of glycol and polyethylene glycol, for example, prevent the adhering of lacquer on the surface of the product. Emulsions, for their part, conduct heat poorly, which results in unevenly hardened pieces and overheating. Heating causes splitting of the pieces, and dimensional changes and burning. Process control is also complicated by the uneven hardening of the chemical, in which case the produced precipitates and other such impurities will hamper the operation of the process apparatus, and the burrs from the slow process will complicate the further treatment of the pieces. The object of the present invention is to provide a simple, environment-friendly, occupationally safe, and inexpensive method for the hardening of wood material by impregnating it with a polymerizable substance and by polymerizing the polymerizable substance by means of heat. The method according to the invention is thus mainly characterized in that the polymerizable substance is polymerized while the wood material is immersed in hot water. It has thus been realized that liquid water is an excellent medium when heating an impregnated wood material to the polymerization temperature and when cooling an exothermal polymerization reaction during polymerization. In the method according to the invention, the wood material is first impregnated with a polymerizable substance. The wood material used may be any wood material, but hardening is most useful with soft species of timber, such as coniferous species. The impregnation is by any known impregnation method, for example by forcing a liquid polymerizable substance into the wood cell tissue by elevated pressure or by impregnating the cell tissue with it by means of a vacuum. Overpressure and vacuum can also be combined in a suitable manner to achieve impregnation. When a wood material has been impregnated with a polymerizable substance, the impregnated wood material is contacted with water before the polymerization. The impregnated wood material is immersed in hot water. The temperature of the water is 40-200 °C, preferably 80-150 °C. It is also advantageous if the water and the impregnated wood material immersed in it are maintained at an overpressure. In this case the overpressure is used for preventing the water from boiling and for preventing the polymerizable substance from evacuating from the wood material. The said overpressure preferably exceeds the vapor pressure at the temperature of the water and is in that case at minimum approx. 3 bar. The polymerization temperature can be adjusted by heating or cooling the water in which the impregnated wood material is immersed. The water in which the impregnated wood material is immersed can be circulated in a substantially closed system in order to carry out heat exchange, cleaning operations, etc. The polymerization is preferably carried out in an inert gas, such as nitrogen gas, the nitrogen gas preventing the oxygen of air from interfering with the polymerization reaction. It is thus advisable to use nitrogen gas already at the impregnation stage. After the impregnation and before the polymerization it is advantageous to rinse the impregnated wood material, and preferably the whole process apparatus, with cold water. Cold water will rinse off excess chemicals from the surfaces of the pieces and will protect the process apparatus so that the actuating devices and pipes are not clogged with polymerizing substances. Polymerization can be initiated by feeding the water, which is hot and preferably pressurized, into the midst of the impregnated wood material. The water being pressurized, of course, requires that the polymerization is carried out in a pressure vessel and that an inert gas such as nitrogen has first been directed into the vessel to generate overpressure. In this manner the impregnated wood material, in part or wholly immersed in water, is heated in an inert atmosphere to a temperature at which polymerization starts. As known, the polymerization reaction is highly exothermal and may cause overheating which will destroy both the wood material and the forming polymer. Overheating caused by the polymerization reaction can be prevented preferably by cooling the water in which the impregnated wood material is immersed. It is, of course, advantageous to use effective mixing and/or circulation via heat exchangers. It is also preferable to circulate the hot water, pressurized, via filters for a sufficiently long time to complete the polymerization. The pressure of the inert gas, such as nitrogen, must be adjusted so that the water will not boil at the temperature concerned. When the polymerization has been brought to completion, the hot water and possibly the pressure are removed and the wood material product is rinsed preferably with cold water. By means of such rinsing, any plastic precipitates formed and other impurities are removed from the surfaces of both the wood material and the polymerization apparatus. The actuating devices return to their correct dimensions, and polymerization which would cause clogging will no longer occur in them. One important advantage of the invention is indeed that it is possible to use the same inexpensive liquid both for the wash and for the heat transfer during the reaction. The advantages of the water cycling system mentioned above include that only an amount of water equal to that leaving in steam, precipitate and the product is fed into the process. The process is thus a substantially closed impregnation and hardening process. The formed plastic precipitate and impurities are removed by filtering and they can be burned together with, for example, saw dust and bark. Any chemical residues produced in the cold water cycle are decanted from the water and are recycled. The residues in the hot-water vessel can be reduced by adding into the hot water, for example overnight, a catalytic agent to precipitate the plastic on the bottom of the vessel, from which it is filtered and transferred for burning. In the method according to the invention the wood material can be impregnated with the polymerizable substance as such or in the form of a solution, dispersion or other mixture. What is most important is that the polymerizable substance is absorbed into the cell tissue of the wood material. According to one embodiment, an agent which regulates the viscosity of the polymerizable substance is mixed with it. The polymerizable substance may in principle be any material which will polymerize or become bridged into a final polymer product in the midst of the wood material. It may be a monomer, a prepolymer, or a mixture of a prepolymer and a monomer. As examples of monomers should be mentioned acrylic monomers such as acrylic esters, methacrylic esters, acrylonitriles, and acrylamides; styrene monomers such as styrene, α-methyl styrene and divinyl benzene; vinyl and vinylidene monomers such as vinyl halides, vinylidene halides, vinyl ethers and vinyl esters; aldehyde monomers such as formaldehyde and acetaldehyde, cyclic ether oxides such as ethylene oxide and propylene oxide; cyclic lactames and lactones. Thermosetting polymerizable substances are in general prepolymers. They include phenol, urea, and melamine formaldehyde products, polyurethanes, epoxides, silicones, and unsaturated polyesters. Some preferred ethylenically unsaturated monomers to be mentioned are the acrylic, styrene and vinyl polymers mentioned above. Prepolymers include the prepolymer of an unsaturated polyester, actually a hardenable mixture of a polyester prepolymer and a styrene or acrylic monomer. When it is desired to harden wood materials by polymerizing a substance with which they have been impregnated, it is preferable to add a bridging reagent to the polymerizable substance. This bridging reagent may be an ordinary monomer, which forms a chain between prepolymers, or a multifunction monomer which, acting as a mer, reacts further and links polymer chains to each other. As mentioned above, the polymerization is preferably carried out in the presence of a catalyst. When unsaturated monomers and/or prepolymers are used, the catalyst is a so-called initiator, such as a radical initiator, an anionic initiator or a cationic initiator. The most important of these initiators is the radical initiator. Some typical radical initiators are peroxides, such as diacyl peroxides, acetyl-alkyl-sulfonyl peroxides, dialkyl peroxydicarbonates, tert-alkyl peroxyesters, OO-tert-alkyl-0-alkyl monomer peroxydicarbonates, di-(tert-alkyl-peroxy)ketals, di-tert-alkyl peroxides, tert-alkyl hydroxides, ketone peroxides, and silyl peroxides. The peroxides, which in polymerization decompose into initiating radicals, are listed above according to their decreasing decomposition tendency. Typical peroxides are di-n-propyl peroxydicarbonate, dilayroyl peroxide, diacetyl peroxide, dibenzoyl peroxide, dicumyl peroxide, and di-tert-butyl peroxide. Other radical initiators include azo-type initiators, such as 2,2'-azobis-[2,4-dimethyl]pentanenitrile, 2,2'-azobis-isobutyronitrile, and 1-(tert-amylazo)cyclohexanecarbonitrile. In the present invention it is also possible to use so-called photoinitiators, which under the effect of light decompose into radicals which initiate polymerization. The present invention may also be combined with state-of-the-art methods so that the use of water as the heat transfer medium is combined, for example, with the use of cooling surfaces according to FI Patent 44946 or retarders according to FI Patent 44945. Such combining can be done by an expert in the art, when necessary. Owing to the present invention, pieces of wood will retain their shape well, the hardness will be considerably more even, species of timber previously difficult to impregnate can be converted to products with a higher degree of processing, separate after-machining will not be needed for removing various detaching agents or additives, very little or no evaporation of chemicals will occur in the process, the apparatus will not require a great deal of space, and the apparatus will not be expensive. As a closed system the process will be occupationally safe and the produced waste can be destroyed in a controlled manner by burning. An embodiment example is given below, the sole purpose of the example being to illustrate the present invention. Example Veneers 1230 mm long, 1.0-5 mm thick and approx. 165 mm wide, packed into a package with an intermediate medium, are impregnated, by using vacuum and pressure, with a chemical mixture which contains approx. 90 % methyl methacrylate, approx. 9.5 % a bridging chemical, for example butyl acrylate or trimethylolpropane triacrylate, and as the catalyst 0.5 % benzoyl peroxide or azobis-isobutyronitrile. After impregnation, any excess impregnation solution having left the cylinder, cold water at approx. 5 °C is fed into it, which rinses any loose chemical from the veneer surfaces and produces in the process apparatus a so-called cold shielding bed to prevent polymerization in the pipes and the actuating devices. MMA is lighter than water, and so it can be removed by rinsing and be separated from the water by decanting in a separate vessel. After the cold rinsing, a nitrogen overpressure of approx. 3-5 bar (overpressure) is fed into the impregnation-hardening cylinder and simultaneously the water to be used for the polymerization is heated, pressurized, to a sufficient temperature, i.e. approx. 100-130 °C, whereafter the hot water is fed into the impregnation-hardening cylinder. When the cylinder has been filled, the pressure is raised in the hot container and the cylinder to approx. 5 bar. Thereafter, polymerization is initiated with the pressurized hot water. The polymerization time is dependent on the timber species, the achieved chemical impregnation, the heating temperature, and the heating time. The heating time is approx. 30-40 min. Any plastic precipitate formed during the heating is directed within the cycle to filters, from which it can be removed and directed to burning. After the heating stage, the cylinder is emptied and cold water is fed into it for rinsing and for evening out the thermal expansion of the actuating devices, and as a shield for the actuating devices in order to conduct the heat to the chemicals to decrease the risk of polymerization. After the cold rinsing the veneers are unloaded from the cylinder. The unloaded veneers contain loose water approx. 20-30 %, which is removed by drying either by means of a roller dryer or a press drier. After the drying, the veneers are ready for use as surfacing for various products. When the hardening activity has been interrupted, for example overnight, a catalyst can be fed into the hot-water tank in order to improve the precipitation of the chemicals. The MMA content in the water in the cold-water tank will rise by 1.5 %, whereafter no more MMA will dissolve in the water. Being lighter than water it will rise to the surface in the tank, from where it will be recovered by decanting and be recycled. As a final result of our process, hardness values have been obtained which are as follows as compared with normal wood: Pine normal Brinell hardness 1.6 N/mm 2 and with hot water hardening approx. 6-11 N/mm 2 , varying with respect to the heart-wood; beech normal Brinell hardness is approx. 2.4 N/mm 2 and hardness after treatment 8-12 N/mm 2 ; birch normal Brinell approx. 2.3 N/mm 2 and hardness after treatment 12-14 N/mm 2 ; and aspen normal hardness 1.2 N/mm 2 and hardness after treatment 12-16 N/mm 2 . In most cases the inherent hardness of wood increases approximately 5-10 fold. Scatter is caused by non-homogeneity and poor permeability to liquids.

Claims: A method for the hardening of wood material by impregnating it with a polymerizable substance and by polymerizing the polymerizable substance by means of heat, characterized in that the polymerizable substance is polymerized while the impregnated wood material is immersed in hot water the temperature of which is 40°C to 200°C. A method according to Claim 1, characterized in that the impregnated wood material is immersed in hot water the temperature of which is 80°C to 150°C. A method according to Claim 1 or 2, characterized in that the water and the impregnated wood material immersed in it are maintained at an overpressure. A method according to Claim 3, characterized in that the said overpressure exceeds the vapor pressure of water at the temperature concerned and is preferably at minimum approx. 3 bar. A method according to any of the above claims, characterized in that the polymerization temperature is regulated by heating or cooling the water in which the impregnated wood material is immersed. A method according to any of the above claims, characterized in that the water in which the impregnated wood material is immersed is circulated in a substantially closed cycle for the carrying out of heat exchange, cleaning operations, etc. A method according to any of the above claims, characterized in that the polymerization is carried out in an inert gas, preferably nitrogen gas. A method according to any of the above claims, characterized in that, between the impregnation and the polymerization, the impregnated wood material, and preferably the entire process apparatus, is rinsed with cold water. A method according to any of the above claims, characterized in that the polymerization is initiated by feeding the water, which is hot and preferably pressurized, into the midst of the impregnated wood material. A method according to any of the above claims, characterized in that any overheating caused by the polymerization reaction is prevented by cooling the water in which the impregnated wood material is immersed. A method according to any of the above claims, characterized in that the hot water is removed after the polymerization, and that the wood material product is rinsed with cold water. A method according to any of the above claims, characterized in that mixed with the polymerizable substance there is an agent which controls its viscosity. A method according to any of the above claims, characterized in that the polymerizable substance used is an organic substance which constitutes a phase separate from the water. A method according to any of the above claims, characterized in that the polymerization is carried out by using as the polymerizable substance a monomer, a prepolymer or a mixture of these, and a polymerization catalyst. A method according to Claim 14, characterized in that the monomer and/or prepolymer and the catalyst are selected so that the polymerization takes place through the free-radical mechanism.

Inventors: MANSIKKAMAEKI PAAVO, PAAKKI MARKKU, MANSIKKAMAEKI, PAAVO, PAAKKI, MARKKU, Mansikkamäki, Paavo
Applicants: FINNFOREST OY
Application year: 1994
Main classification: B27K3
IPC codes: B27K5, B27K3